Network Working Group S. Farrell
Internet-Draft Trinity College Dublin
Intended status: Experimental P. Hoffman
Expires: June 29, 2015 VPN Consortium
M. Thomas
Phresheez
December 26, 2014
HTTP Origin-Bound Authentication (HOBA)
draft-ietf-httpauth-hoba-08
Abstract
HTTP Origin-Bound Authentication (HOBA) is a digital signature based
design for an HTTP authentication method. The design can also be
used in Javascript-based authentication embedded in HTML. HOBA is an
alternative to HTTP authentication schemes that require passwords and
therefore avoids all problems related to passwords, such as leakage
of server-side password databases.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on June 29, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Interfacing to Applications (Cookies) . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. The HOBA Authentication Scheme . . . . . . . . . . . . . . . 5
3. Introduction to the HOBA-http Mechanism . . . . . . . . . . . 8
4. Introduction to the HOBA-js Mechanism . . . . . . . . . . . . 9
5. HOBA's Authentication Process . . . . . . . . . . . . . . . . 10
5.1. CPK Preparation Phase . . . . . . . . . . . . . . . . . . 10
5.2. Signing Phase . . . . . . . . . . . . . . . . . . . . . . 10
5.3. Authentication Phase . . . . . . . . . . . . . . . . . . 10
6. Other Parts of the HOBA Process . . . . . . . . . . . . . . . 11
6.1. Registration . . . . . . . . . . . . . . . . . . . . . . 12
6.1.1. Hobareg Definition . . . . . . . . . . . . . . . . . 13
6.2. Associating Additional Keys to an Existing Account . . . 14
6.2.1. Moving private keys . . . . . . . . . . . . . . . . . 15
6.2.2. Human memorable one time password (don't do this one) 15
6.2.3. Out of band URL . . . . . . . . . . . . . . . . . . . 16
6.3. Logging Out . . . . . . . . . . . . . . . . . . . . . . . 16
6.4. Getting a Fresh Challenge . . . . . . . . . . . . . . . . 16
7. Mandatory-to-Implement Algorithms . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8.1. Privacy considerations . . . . . . . . . . . . . . . . . 17
8.2. localStorage Security for Javascript . . . . . . . . . . 17
8.3. Multiple Accounts on One User Agent . . . . . . . . . . . 18
8.4. Injective Mapping for HOBA-TBS . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9.1. HOBA Authentication Scheme . . . . . . . . . . . . . . . 19
9.2. .well-known URI . . . . . . . . . . . . . . . . . . . . . 19
9.3. Algorithm Names . . . . . . . . . . . . . . . . . . . . . 20
9.4. Key Identifier Types . . . . . . . . . . . . . . . . . . 20
9.5. Device Identifier Types . . . . . . . . . . . . . . . . . 20
9.6. Hobareg HTTP Header Field . . . . . . . . . . . . . . . . 21
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Problems with Passwords . . . . . . . . . . . . . . 23
Appendix B. Example . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
HTTP Origin-Bound Authentication (HOBA) is an authentication design
that can be used as an HTTP authentication scheme [RFC7235] and for
Javascript-based authentication embedded in HTML. The main goal of
HOBA is to offer an easy-to-implement authentication scheme that is
not based on passwords, but that can easily replace HTTP or HTML
forms-based password authentication. Deployment of HOBA can reduce
or eliminate password entries in databases, with potentially
significant security benefits.
HOBA is an HTTP authentication mechanism that complies with the
framework for such schemes [RFC7235]. As a JavaScript design, HOBA
demonstrates a way for clients and servers to interact using the same
credentials that are used by the HTTP authentication scheme.
Current username/password authentication methods such as HTTP Basic,
HTTP Digest, and web forms have been in use for many years but are
susceptible to theft of server-side password databases. Instead of
passwords, HOBA uses digital signatures as an authentication
mechanism. HOBA also adds useful features such as credential
management and session logout. In HOBA, the client creates a new
public-private key pair for each host ("web origin" [RFC6454]) to
which it authenticates. These keys are used in HOBA for HTTP clients
to authenticate themselves to servers in the HTTP protocol or in a
Javascript authentication program.
HOBA session management is identical to username/password session
management, with a server-side session management tool or script
inserting a session cookie [RFC6265] into the output to the browser.
Use of TLS for the HTTP session is still necessary to prevent session
cookie hijacking.
HOBA keys are "bare keys", so there is no need for the semantic
overhead of X.509 public key certificates, particularly with respect
to naming and trust anchors. The client public key ("CPK")
structures in HOBA do not have any publicly-visible identifier for
the user who possesses the corresponding private key, nor the web-
origin with which the client is using the CPK.
HOBA also defines some services that are needed for modern HTTP
authentication:
o Servers can bind a CPK with an identifier, such as an account
name. Servers using HOBA define their own policies for binding
CPKs with accounts during account registration.
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o Users are likely to use more than one device or user agent (UA)
for the same HTTP based service, so HOBA gives a way to associate
more than one CPK to the same account, but without having to
register for each separately.
o Logout features can be useful for UAs, so HOBA defines a way to
close a current HTTP "session", and also a way to close all
current sessions, even if more than one session is currently
active from different UAs for the same account.
o Digital signatures can be expensive to compute, so HOBA defines a
way for HTTP servers to indicate how long a given challenge value
is valid, and a way for UAs to fetch a fresh challenge at any
time.
Users are also likely to lose a private key, or the client's memory
of which key pair is associated with which origin, such as when a
user loses the computer or mobile device in which state is stored.
HOBA does not define a mechanism for deleting the association between
an existing CPK and an account. Such a mechanism can be implemented
at the application layer.
1.1. Interfacing to Applications (Cookies)
HOBA can be used as a drop-in replacement for password-based user
authentication schemes used in common web applications. The simplest
way is to (re-)direct the UA to a HOBA "Login" URL and for the
response to a successful HTTP request containing a HOBA signature to
set a session cookie [RFC6265]. Further interactions with the web
application will then be secured via the session cookie, as is
commonly done today.
While cookies are bearer tokens, and thus weaker than HOBA
signatures, they are currently ubiquitously used. If non-bearer
token session continuation schemes are developed in future in the
IETF or elsewhere, then those can interface to HOBA as easily as with
any password based authentication scheme.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234].
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Account: The term "account" is (loosely) used to refer to whatever
data structure(s) the server maintains that are associated with an
identity. That will contain of at least one CPK and a web-origin; it
will also optionally include an HTTP "realm" as defined in the HTTP
authentication specification [RFC7235]. It might also involve many
other non-standard pieces of data that the server accumulates as part
of account creation processes. An account may have many CPKs that
are considered equivalent in terms of being usable for
authentication, but the meaning of "equivalent" is really up to the
server and is not defined here.
Client public key ("CPK"): A CPK is the public key and associated
cryptographic parameters needed for a server to validate a signature.
HOBA-http: We use this term when describing something that is
specific to HOBA as an HTTP authentication mechanism.
HOBA-js: We use this term when describing something that is unrelated
to HOBA-http but is relevant for HOBA as a design pattern that can be
implemented in a browser in JavaScript.
User agent (UA): typically, but not always, a web browser.
User: a person who is running a UA. In this document, "user" does
not mean "user name" or "account name".
Web client: the content and javascript code that run within the
context of a single UA instance (such as a tab in a web browser).
2. The HOBA Authentication Scheme
A UA that implements HOBA maintains a list of web-origins and realms.
The UA also maintains one or more client credentials for each web-
origin/realm combination for which it has created a CPK.
On receipt of a challenge (and optional realm) from a server, the
client marshals an HOBA to-be-signed (TBS) blob that includes a
client generated nonce, the web-origin, the realm, an identifier for
the CPK and the challenge string; and signs that blob with the
private key corresponding to the CPK for that web-origin. The
formatting chosen for this TBS blob is chosen so as to make server-
side signature verification as simple as possible for a wide range of
current server tooling.
Figure 1 specifies the ABNF for the signature input. The term
"unreserved" means that the field does not have a specific format
defined and allows the characters specified in Section 2.3 of
[RFC3986].
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HOBA-TBS = len ":" nonce
len ":" alg
len ":" origin
len ":" [ realm ]
len ":" kid
len ":" challenge
len = 1*DIGIT
nonce = 1*base64urlchars
alg = 1*2DIGIT
origin = scheme "://" authority ":" port
; scheme etc are from RFC 3986
realm = unreserved
; realm is to be treated as in Section 2.2 of RFC 7235
kid = 1*base64urlchars
challenge = 1*base64urlchars
; Characters for Base64URL encoding from Table 2 of RFC 4648
; all of which are US-ASCII (see RFC 20)
base64urlchars = %x30-39 ; Digits
/ %x41-5A ; Uppercase letters
/ %x61-7A ; Lowercase letters
/ "-" / "_" / "=" ; Special characters
Figure 1: To-be-signed data for HOBA
The fields above contain the following:
o len: Each field is preceeded by the number of octets of the
following field, expressed as a decimal number in ASCII [RFC0020].
Lengths are separated from field values by a colon character. So
if a nonce with the value "ABCD" were used then that would be
preceeded by "4:" (see the example in Appendix B for detail).
o nonce: is a random value chosen by the UA and MUST be base64url
encoded before being included in the HOBA-TBS value. (base64url
encoding is defined in [RFC4648].) UAs MUST be able to use at
least 32 bits of randomness in generating a nonce. UAs SHOULD be
able to use 64 or more bits of randomness for nonces.
o alg: specifies the signature algorithm being used encoded as an
ASCII character as defined in Section 9.3. RSA-SHA256 MUST be
supported, RSA-SHA1 MAY be supported. The IANA registered
algorithm values are encoded as ASCII numbers; for example, the
encoding of RSA-SHA256 is 0x30.
o origin: is the web origin expressed as the concatenation of the
scheme, authority and port from [RFC3986]. These are not base64
encoded as they will be most readily available to the server in
plain text. For example, if accessing the URL "https://
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www.example.com:8080/foo" then the bytes input to the signature
process will be "https://www.example.com:8080". There is no
default for the port number, and the port number MUST be present.
o realm: is similarly just a string with the syntactic restrictions
defined in [RFC7235]. If no realm is specified for this
authentication then this is absent, but is preceeded by a length
of zero ("0:"). Recall both sides know when this needs to be
there independent of the encoding via a zero length.
o kid: is a key identifier - this MUST be a base64url encoded value
that is presented to the server in the HOBA client result (see
below).
o challenge: MUST be a base64url encoded challenge value that the
server chose to send to the client. The challenge MUST be chosen
so that it is infeasible to guess, and SHOULD be indistinguishable
from (the base64url encoding of) an at least 128-bit long random
string.
The HOBA-TBS string is the input to the client's signing process, but
is not itself sent over the network since some fields are already
inherent in the HTTP exchange. The challenge however is sent over
the network so as to reduce the amount of reduce the amount of state
that needs to be maintained by servers. (One form of stateless
challenge might be a ciphertext that the server decrypts and checks,
but that is an implementation detail.) The value that is sent over
the network by the UA is the HOBA "client result" which we now
define.
The HOBA "client result" is a dot-separated string that includes the
signature and is sent in the HTTP Authorizaion header field value
using the value syntax defined in Figure 2. The "sig" value is the
base64url encoded version of the binary output of the signing
process. The kid, challenge and nonce are as defined above and are
also base64url encoded.
HOBA-RES = kid "." challenge "." nonce "." sig
sig = 1*base64urlchars
Figure 2: HOBA Client Result value
The HOBA scheme is far from new, for example, the basic idea is
pretty much identical to the first two messages from "Mechanism R" on
page 6 of [MI93] which predates HOBA by 20 years.
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3. Introduction to the HOBA-http Mechanism
An HTTP server that supports HOBA authentication includes the "HOBA"
auth-scheme value in a WWW-Authenticate header field when it wants
the client to authenticate with HOBA. Note that the HOBA auth-scheme
might not be the only one that the server includes in a WWW-
Authenticate header.
The HOBA scheme has two REQUIRED attributes (challenge and max-age)
and one OPTIONAL attribute (realm):
o The "challenge" attribute MUST be included. The challenge is the
string made up of the base64url encoded octets that the server
wants the client to sign in its response. The challenge MUST be
unique for every HTTP 401 response in order to prevent replay
attacks from passive observers.
o A "max-age" attribute MUST be included that specifies the number
of seconds from the time the HTTP response is emitted for which
responses to this challenge can be accepted for example "max-age:
10" would indicate ten seconds. If max-age is set to zero, then
that means that only one signature will be accepted for this
challenge.
o A "realm" attribute MAY be included to indicate the scope of
protection in the manner described in HTTP/1.1, Part 7 [RFC7235].
The "realm" attribute MUST NOT appear more than once.
When the "client response" is created, the UA encodes the HOBA
client-result and returns that in the Authorization header. The
client-result is a string matching the HOBA-RES production in Figure
2 as an auth-param with the name "result".
The server MUST check the cryptographic correctness of the signature
based on a public key it knows for the kid in the signatures, and if
the server cannot do that, or if the signature fails cryptographic
checks, then validation has failed. The server can use any
additional mechanisms to validate the signature. If the validation
fails, or if the server chooses reject the signature for any reason
whatsoever, the server fails the request with a 401 Unauthorized HTTP
response.
The server MUST check that the same web origin is used in all of the
server's TLS server certificates, the URL being accessed and the HOBA
signature. If any of those checks fail, the server treats the
signature as being cryptographically incorrect.
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Note that a HOBA signature is good for however long a non-zero max-
age parameter allows. This means that replay is possible within the
time window specified by the "max-age" value chosen by the server.
Servers can attempt to detect any such replay (via caching if they so
choose) and MAY react to such replays by responding with a second (or
subsequent) 401-status HTTP response containing a new challenge.
UAs MAY optimise their use of challenges by pre-fetching a challenge
value, for example after (max-age)/2 seconds have elapsed, using the
".well-known/hoba/getchal" scheme described later in this document.
This also allows for pre-calculation of HOBA signatures, if that is
required in order to produce a responsive user interface.
4. Introduction to the HOBA-js Mechanism
Web sites using JavaScript can also perform origin-bound
authentication without needing to involve the HTTP layer, and by
inference not needing HOBA-http support in browsers. HOBA-js is not
an on-the-wire protocol like HOBA-http is: instead, it is a design
pattern that can be realized completely in JavaScript served in
normal HTML pages.
One element is required for HOBA-js: localStorage (see http://
www.w3.org/TR/webstorage/) from HTML5 can be used for persistent key
storage. For example, an implementation would store a dictionary
account identifier, public key and private key tuples in the origin's
localStorage for subsequent authentication requests. How this
information is actually stored in localStorage is an implementation
detail. This type of key storage relies on the security properties
of the same-origin policy that localStorage enforces. See the
security considerations for discussion about attacks on localStorage.
Note that IndexedDB (See http://www.w3.org/TR/IndexedDB/) is an
alternative to localStorage that can also be used here.
Because of JavaScript's same-origin policy, scripts from subdomains
do not have access to the same localStorage that scripts in their
parent domains do. For larger or more complex sites, this could be
an issue that requires enrollment into subdomains, which could be a
hassle for users. One way to get around this is to use session
cookies because they can be used across subdomains. That is, with
HOBA-js, the user might log in using a single well-known domain, and
then the server uses session cookies to navigate around a site.
Another element will be highly desirable for HOBA-js when it becomes
available: WebCrypto (see http://www.w3.org/TR/WebCryptoAPI). In
lieu of WebCrypto, JavaScript crypto libraries can be employed with
the known deficiencies of their pseudo-random number generators and
the general immaturity of those libraries.
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5. HOBA's Authentication Process
This section describes how clients and servers use HOBA for
authentication. The interaction between an HTTP client and HTTP
server using HOBA happens in three phases: the CPK preparation phase,
the signing phase, and the authentication phase. This section also
covers the actions that give HOBA similar user features as today's
passwords have.
5.1. CPK Preparation Phase
In the CPK preparation phase, the client determines if it already has
a CPK for the web-origin with which it needs to authenticate. If the
client has a CPK, the client will use it; if the client does not have
a CPK, it generates one in anticipation of the server asking for one.
5.2. Signing Phase
In the signing phase, the client connects to the server, the server
asks for HOBA-based authentication, and the client authenticates by
signing a blob of information as described in the previous sections.
5.3. Authentication Phase
The authentication phase is completely dependent on the policies and
practices of the server. That is, this phase involves no
standardized protocol in HOBA-http; in HOBA-js, there is no suggested
interaction template.
In the authentication phase, the server uses the key identifier (kid)
to determine the CPK from the signing phase and decides if it
recognizes the CPK. If the server recognizes the CPK, the server may
finish the client authentication process.
If this stage of the process involves additional information for
authentication, such as asking the user which account she wants to
use (in the case where a UA is used for multiple accounts on a site),
the server can prompt the user for account identifying information or
the user could choose based on HTML offered by the server before the
401 response is triggered. None of this is standardized: it all
follows the server's security policy and session flow. At the end of
this, the server probably assigns or updates a session cookie for the
client.
During the authentication phase, if the server cannot determine the
correct CPK, it could use HTML and JavaScript to ask the user if they
are really a new user or want to associate this new CPK with another
CPK. The server can then use some out-of-band method (such as a
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confirmation email round trip, SMS, or an UA that is already
enrolled) to verify that the "new" user is the same as the already-
enrolled one. Thus, logging in on a new user agent is identical to
logging in with an existing account.
If the server does not recognize the CPK the server might send the
client through a either a join or login-new-UA (see below) process.
This process is completely up to the server, and probably entails
using HTML and JavaScript to ask the user some questions in order to
assess whether or not the server wants to give the client an account.
Completion of the joining process might require confirmation by
email, SMS, Captcha, and so on.
Note that there is no necessity for the server to initiate a joining
or login process upon completion of the signing phase. Indeed, the
server may desire to challenge the UA even for unprotected resources
and set a session cookie for later use in a join or login process as
it becomes necessary. For example, a server might only want to offer
an account to someone who had been to a few pages on the web site; in
such a case, the server could use the CPK from an associated session
cookie as a way of building reputation for the user until the server
wants the user to join.
6. Other Parts of the HOBA Process
The authentication process is more than just the act of
authentication. In password-based authentication and HOBA, there are
other processes that are needed both before and after an
authentication step. This section covers those processes. Where
possible, it combines practices of HOBA-http and HOBA-js; where that
is not possible, the differences are called out.
All HOBA interactions other than those defined in Section 5 MUST be
performed in TLS-protected sessions ([RFC5246]). If the current HTTP
traffic is not running under TLS, a new session is started before any
of the actions described here are performed.
HOBA-http uses a well-known URL [RFC5785] "hoba" as a base URI for
performing many tasks: "https://www.example.com/.well-known/hoba".
These URLs are based on the name of the host that the HTTP client is
accessing.
There are many use cases for these URLs to redirect to other URLs: a
site that does registration through a federated site, a site that
only does registration under HTTPS, and so on. Like any HTTP client,
HOBA-http clients have to be able to handle redirection of these
requests. However, as that would potentially cause security issues
when a re-direct brings the client to a different web origin, servers
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implementing HOBA-http SHOULD NOT re-direct to a different web origin
from below .well-known/hoba URLs. The above is considered sufficient
to allow experimentation with HOBA, but if at some point HOBA is
placed on the standards track then a full analysis of off-origin re-
directions would need to be documented.
6.1. Registration
Normally, a registration (also called "joining") is expected to
happen after a UA receives a 401 response for a web-origin and realm
(for HOBA-http) or on demand (for HOBA-js) for which it has no
associated CPK. The process of registration for a HOBA account on a
server is relatively light-weight. The UA generates a new key pair,
and associates it with the web-origin/realm in question.
Note that if the UA has a CPK associated with the web-origin, but not
for the realm concerned, then a new registration is REQUIRED. If the
server did not wish for that outcome, then it ought to use the same
or no realm.
The registration message for HOBA-http is sent as a POST message to
the URL ".well-known/hoba/register" with an HTML form (x-www-form-
encoded) (see http://www.w3.org/TR/2014/REC-html5-20141028/forms.html
#url-encoded-form-data) described below; The registration message for
HOBA-js can be in any format specified by the server, but it could be
the same as the one described here for HOBA-http. It is up to the
server to decide what kind of user interaction is required before the
account is finally set up. When the server's chosen registration
flow is completed successfully the server MUST add a Hobareg HTTP
header (see Section 6.1.1) to the HTTP response message that
completes the registration flow.
The registration message sent to server has one mandatory field (pub)
and some optional fields that allow the UA to specify the type and
value of key and device identifiers that the UA wishes to use.
o pub: is a mandatory field containing the PEM formatted public key
of the client. See Appendix C of [RFC6376] for an example of how
to generate this key format.
o kidtype: contains the type of key identifier, this is a numeric
value intended to contain one of the values from Section 9.4. If
this is not present then the mandatory-to-implement hashed public
key option MUST be used.
o kid: contains the key identifier as a base64url encoded string
that is of the type indicated in the kidtype. If the kid is a
hash of a public key then the correct (base64url encoded) hash
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value MUST be provided and the server SHOULD check that and refuse
the registration if an incorrect value was supplied.
o didtype: specifies a kind of device identifier intended to contain
one of the values from Section 9.5, if absent then the "string"
form of device identifier MUST be used.
o did: a UTF8 string that specifies the device identifier. This can
be used to help a user be confident that authentication has
worked, e.g., following authentication some web content might say
"You last logged in from device 'did' at time T."
Note that replay of registration (and other HOBA) messages is quite
possible. That however can be counteracted if challenge freshness is
ensured. See Section 2 for details. Note also that with HOBA-http
the HOBA signature does not cover the POST message body. If that is
required then HOBA-JS may be a better fit for registration and other
account management actions.
6.1.1. Hobareg Definition
Since registration can often be a multi-step process, e.g. requiring
a user to fill in contact details, the initial response to the HTTP
POST message defined above may not be the end of the registration
process even though the HTTP response has a 200 OK status. This
creates an issue for the UA since, during the registration process
(e.g., while dealing with interstitial pages), the UA doesn't yet
know whether the CPK is good for that web origin or not.
For this reason the server MUST add a header field to the response
message when the registration has succeded to indicate the new state.
The header to be used is "Hobareg" and the value when registration
has succeeded is to be "regok". When registration is in an
intermediate state (e.g. on an HTTP response for an interstitial
page) the server MAY add this header with a value of "reginwork".
See Section 9.6 for the relevant IANA registration of this header
field.
For interstitial pages, the client MAY include a HOBA Authorization
header. This is not considered a MUST as that might needlessly
complicate client implementations but is noted here in case a server
implementer assumes that all registration messages contain a HOBA
Authorization header.
Hobareg-val = "regok" / "reginwork"
Figure 3: Hobareg Header Field Definition
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Figure 3 provides an ABNF definition for the values allowed in the
Hobareg header field. Note that these (and the header field name)
are case insensitive. Section 8.3.1 of [RFC7231] calls for
documenting the following details for this new header field:
o Only one single value is allowed in a Hobareg header field.
Should more than one (a list) be encountered or any other ANBF-
invalid value, that SHOULD be interpreted as being the same as
"reginwork."
o The Hobareg header field can only be used in HTTP responses.
o Since Hobareg is only meant for responses it ought not appear in
requests.
o The HTTP response code does affect the interpretation of Hobareg.
Registration is only considered to have succeeded if the regok
value is seen in a 2xx response. 4xx and other errors means that
registration has failed regardless of the value of Hobareg seen.
The request method has no influence on the interpretation of
Hobareg.
o Intermediaries never insert, delete or modify a Hobareg header
field.
o As a response-only header field, it is not appropriate to list a
Hobareg in a Vary response header field.
o Hobareg is allowed in trailers.
o As a response-only header field, Hobareg will not be preserved
across re-directs.
o Hobareg itself discloses little security or privacy sensitive
information. If an attacker can somehow detect that a Hobareg
header field is being added, then that attacker would know that
the UA is in the process of registration which could be
significant. However, it is likely that the set of messages
between the UA and server would expose this information in many
cases, regardless of whether or not TLS is used. Using TLS is
still however a good plan.
6.2. Associating Additional Keys to an Existing Account
From the user perspective, the UA having a CPK for a web origin will
often appear to be the same as having a way to sign in to an account
at that web site. Since users often have more than one UA, and since
the CPKs are, in general, UA-specific, that raises the question of
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how the user can sign in to that account from different UAs. And
from the server perspective that turns into the question of how to
safely bind different CPKs to one account. In this section, we
describe some ways in which this can be done, as well as one way in
which this ought not be done.
Note that the context here is usually that the user has succeeded in
registering with one or more UAs (for the purposes of this section we
call this "the first UA" below) and can use HOBA with those, and the
user is now adding another UA. The newest UA might or might not have
a CPK for the site in question. Since it is in fact trivial, we
assume that the site is able to put in place some appropriate
quicker, easier registration for a CPK for the newest UA. The issue
then becomes one of binding the CPK from the newest UA with those of
other UAs bound to the account.
6.2.1. Moving private keys
It is common for a user to have multiple UAs, and to want all those
UAs to be able to authenticate to a single account. One method to
allow a user who has an existing account to be able to authenticate
on a second device is to securely transport the private and public
keys and the origin information from the first device to the second.
If this approach is taken, then there is no impact on the HOBA-http
or HOBA-js so this is a pure UA implementation issue and not
discussed further.
6.2.2. Human memorable one time password (don't do this one)
It will be tempting for implementers to use a human-memorable one-
time password (OTP) in order to "authenticate" binding CPKs to the
same account. The workflow here would likely be something along the
lines of some server administrative utility generating a human
memorable OTP such as "1234" and sending that to the user out of band
for the user to enter at two web pages each authenticated via the
relevant CPK. While this seems obvious enough and could even be
secure enough in some limited cases, we consider that this is too
risky to use in the Internet and so servers SHOULD NOT provide such a
mechanism. The reason this is so dangerous is that it would be
trivial for an automated client to guess such tokens and "steal" the
binding intended for some other user. At any scale, there would
always be some in-process bindings so that even with only a trickle
of guesses (and hence not being detectable via message volume) an
attacker would have a high probability of succeeding in registering a
binding with the attacker's CPK.
This method of binding CPKs together is therefore NOT RECOMMENDED.
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6.2.3. Out of band URL
One easy binding method is to simply provide a web page where, using
the first UA, the user can generate a URL (containing some
"unguessable" cryptographically generated value) that the user then
later de-references on the newest UA. The user could e-mail that URL
to herself for example, of the web server accessed at the first UA
could automatically do that.
Such a URL SHOULD contain at least the equivalent of 128 bits of
randomness.
6.3. Logging Out
The user can tell the server it wishes to log out. With HOBA-http,
this is done by sending a HOBA-authenticated POST message to the URL
".well-known/hoba/logout" on the site in question. The UA SHOULD
also delete session cookies associated with the session so that the
user's state is no longer "logged in."
The server MUST NOT allow TLS session resumption for any logged out
session.
The server SHOULD also revoke or delete any cookies associated with
the session.
6.4. Getting a Fresh Challenge
The UA can get a "fresh" challenge from the server. In HOBA-http, it
sends a POST message to ".well-known/hoba/getchal". If successful,
the response response MUST contain a fresh (base64url encoded) HOBA
challenge for this origin in the body of the response. Whitespace in
the response MUST be ignored.
7. Mandatory-to-Implement Algorithms
RSA-SHA256 MUST be supported. RSA-SHA1 MAY be used. RSA modulus
lengths of at least 2048 bits SHOULD be used. RSA is defined in
Section 8.2 of [RFC3447], and SHA-1 and SHA-256 are defined in [SHS].
8. Security Considerations
If key binding was server-selected then a bad actor could bind
different accounts belonging to the user from the network with
possible bad consequences, especially if one of the private keys was
compromised somehow.
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Binding my CPK with someone else's account would be fun and
profitable so SHOULD be appropriately hard. In particular URLs or
other values generated by the server as part of any CPK binding
process MUST be hard to guess, for whatever level of difficulty is
chosen by the server. The server SHOULD NOT allow a random guess to
reveal whether or not an account exists.
When the max-age parameter is not zero, then a HOBA signature has a
property that is like a bearer token for the relevant number of
seconds: it can be replayed for a server-selected duration.
Similarly, for HOBA-js, signatures might be replayable depending on
the specific implementation. The security considerations of
[RFC6750] therefore apply in any case where the HOBA signature can be
replayed. Server administrators can set the max-age to the minimum
acceptable value in such cases, which would often be expected to be
just a few seconds. There seems to be no reason to ever set the max-
age more than a few minutes; the value ought also decrease over time
as device capabilities improve. The administrator will most likely
want to set the max-age to something that is not too slow on the
slowest signing device that is significant for that site.
8.1. Privacy considerations
HOBA does impact to some extent on privacy and could be considered to
represent a super-cookie to the server, or to any entity on the path
from UA to HTTP server that can see the HOBA signature. This is
because we need to send a key identifier as part of the signature and
that will not vary for a given key. For this reason, and others, it
is strongly RECOMMENDED to only use HOBA over server-authenticated
TLS and to migrate web sites using HOBA to only use "https" URLs.
UAs SHOULD provide users a way to manage their CPKs. Ideally, there
would be a way for a user to maintain their HOBA details for a site
while at the same time deleting other site information such as
cookies or non-HOBA HTML5 LocalStorage. However, as this is likely
to be complex and appropriate user interfaces counter intutitive, we
expect that UAs that implement HOBA will likely treat HOBA
information as just some more site data, that would disappear should
the user choose to "forget" that site.
8.2. localStorage Security for Javascript
The use of localStorage (likely with a non-WebCrypto implementation
of HOBA-js) will undoubtedly be a cause for concern. localStorage
uses the same-origin model which says that the scheme, domain and
port define a localStorage instance. Beyond that, any code executing
will have access to private keying material. Of particular concern
are XSS attacks which could conceivably take the keying material and
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use it to create UAs under the control of an attacker. But XSS
attacks are in reality across the board devastating since they can
and do steal credit card information, passwords, perform illicit
acts, etc, etc. It's not clear that we introduce unique threats from
which clear text passwords don't already suffer.
Another source of concern is local access to the keys. That is, if
an attacker has access to the UA itself, they could snoop on the key
through a javascript console, or find the file(s) that implement
localStorage on the host computer. Again it's not clear that we are
worse in this regard because the same attacker could get at browser
password files, etc too. One possible mitigation is to encrypt the
keystore with a password/pin the user supplies. This may sound
counter intuitive, but the object here is to keep passwords off of
servers to mitigate the multiplier effect of a large scale compromise
ala LinkedIn because of shared passwords across sites.
It's worth noting that HOBA uses asymmetric keys and not passwords
when evaluating threats. As various password database leaks have
shown, the real threat of a password breach is not just to the site
that was breached, it's all of the sites a user used the same
password on too. That is, the collateral damage is severe because
password reuse is common. Storing a password in localStorage would
also have a similar multiplier effect for an attacker, though perhaps
on a smaller scale than a server-side compromise: one successful
crack gains the attacker potential access to hundreds if not
thousands of sites the user visits. HOBA does not suffer from that
attack multiplier since each asymmetric key pair is unique per site/
UA/user.
8.3. Multiple Accounts on One User Agent
A shared UA with multiple accounts is possible if the account
identifier is stored along with the asymmetric key pair binding them
to one another. Multiple entries can be kept, one for each account,
and selected by the current user. This, of course, is fraught with
the possibility for abuse, since a server is potentially enrolling
the device for a long period and the user may not want to have to be
responsible for the credential for that long. To alleviate this
problem, the user can request that the credential be erased from the
browser. Similarly, during the enrollment phase, a user could
request that the key pair only be kept for a certain amount of time,
or that it not be stored beyond the current browser session.
8.4. Injective Mapping for HOBA-TBS
The repeated length fields in the HOBA-TBS structure are present in
order to ensure that there is no possibility that the catenation of
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different input values can cause confusion that might lead to an
attack, either against HOBA as specified here, or else an attack
against some other protocol that re-used this to-be-signed structure.
Those fields ensure that the mapping from input fields to the HOBA-
TBS string is an injective mapping.
9. IANA Considerations
IANA is requested to make registrations and create new registries as
described below.
For all new registries requested by this document, please place those
beneath a new "HTTP Origin-Bound Authentication (HOBA) Parameters"
category.
9.1. HOBA Authentication Scheme
Please register a new scheme in the HTTP Authentication Scheme
Registry registry as follows:
Authentication Scheme Name: HOBA
Pointer to specification text: Section 3 of [[ this document ]]
Notes (optional): The HOBA scheme can be used with either HTTP
servers or proxies. When used in response to a 407 Proxy
Authentication Required indication, the appropriate proxy
authentication header fields are used instead, as with any other HTTP
authentication scheme.
9.2. .well-known URI
Please register a new .well-known URI in the Well-Known URIs registry
as described below.
URI suffix: hoba
Change controller: IETF
Specification document(s): Section 6 of [[ this document ]]
Related information: N/A
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9.3. Algorithm Names
Please create a new HOBA signature algorithms registry as follows,
with the specification required rule for updates. New HOBA signature
algorithms SHOULD be in use with other IETF standards track protocols
before being added to this registry.
Number Meaning
----------- --------------------------------------------
0 RSA-SHA256
1 RSA-SHA1
RSA is defined in Section 8.2 of [RFC3447], and SHA-1 and SHA-256 are
defined in [SHS].
For this registry the number column should contain a small positive
integer. Following the ABNF above, the maximum value for this is
decimal 99.
9.4. Key Identifier Types
Please create a new HOBA Key Identifier Types registry as follows,
with the specification required rule for updates.
Number Meaning
----------- --------------------------------------------
0 a hashed public key [RFC6698]
1 a URI [RFC3986]
2 an unformatted string, at the user's/UA's whim
For the number 0, hashed public keys are as done in DANE. [RFC6698]
For this registry the number column should contain a small positive
integer.
9.5. Device Identifier Types
Please create a new HOBA Device Identifier Types registry as follows,
with the specification required rule for updates.
Number Meaning
----------- --------------------------------------------
0 an unformatted string, at the user's/UA's whim
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For this registry the number column should contain a small positive
integer.
9.6. Hobareg HTTP Header Field
Please register a new identifier in the Permanent Message Header
Field Names registry as described below.
Header field name: Hobareg
Applicable protocol: HTTP (RFC 7230)
Status: Experimental
Author/Change controller: IETF
Specification document(s): Section 6.1.1 of [[ this document ]]
Related information: N/A
10. Implementation Status
[[ Note to RFC editor - please delete this section before
publication. ]]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC6982].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC6982] "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, by considering the running code as evidence of valuable
experimentation and feedback that has made the implemented protocols
more mature. It is up to the individual working groups to use this
information as they see fit".
At the time of writing there are three known implementations.
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One done by Stephen Farrell of HOBA-http and a HOBA-JS variant
implements the current (-08) version of HOBA and is available from
https://hoba.ie/ which site also includes a demonstration of HOBA.
There is another implementation by Michael Thomas of an HOBA-JS
variant.
The most recent (Dec 2014) implementation is by Portugal Telecom
and is available from https://github.com/razevedo/hoba-
authentication
11. Acknowledgements
Thanks to the following for good comments received during the
preparation of this specification: David Black, Amos Jeffries,
Benjamin Kaduk, Watson Ladd, Matt Lepinski, Ilari Liusvaara, James
Manger, Alexey Melnikov, Yoav Nir, Mark Nottingham, Julian Reschke,
Michael Richardson, Yaron Sheffer, and Michael Sweet. All errors and
stupidities are of course the editors' fault.
12. References
12.1. Normative References
[RFC0020] Cerf, V., "ASCII format for network interchange", RFC 20,
October 1969.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, April
2010.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
2011.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750, October 2012.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content", RFC 7231, June 2014.
[RFC7235] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Authentication", RFC 7235, June 2014.
[SHS] NIST, , "Secure Hash Standard (SHS), FIPS PUB 180-4", NIST
Special Publications , March 2012.
12.2. Informative References
[MI93] Mitchell, and Thomas, "Standardising Authentication
Protocols Based on Public-Key Techniques.", Journal of
Computer Security 2 (1993): 23-36. , 1993.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
[RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
September 2011.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982, July
2013.
[bonneau] Bonneau, , "The science of guessing: analyzing an
anonymized corpus of 70 million passwords.", IEEE
Symposium on Security and Privacy , 2012.
Appendix A. Problems with Passwords
By far the most common mechanism for web authentication is passwords
that can be remembered by the user, called "human-memorable
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passwords". There is plenty of good research on how users typically
use human-memorable passwords (e.g. see [bonneau]), but some of the
highlights are that users typically try hard to reuse passwords on as
many web sites as possible, and that web sites often use either email
addresses or users' names as the identifier that goes with these
passwords.
If an attacker gets access to the database of memorizable passwords,
that attacker can impersonate any of the users. Even if the breach
is discovered, the attacker can still impersonate users until every
password is changed. Even if all the passwords are changed or at
least made unusable, the attacker now possesses a list of likely
username/password pairs that might exist on other sites.
Using memorizable passwords on unencrypted channels also poses risks
to the users. If a web site uses either the HTTP Basic
authentication method, or an HTML form that does no cryptographic
protection of the password in transit, a passive attacker can see the
password and immediately impersonate the user. If a hash-based
authentication scheme such as HTTP Digest authentication is used, a
passive attacker still has a high chance of being able to determine
the password using a dictionary of known passwords.
Note that passwords that are not human-memorable are still subject to
database attack, though are of course unlikely to be re-used across
many systems. Similarly, database attacks of some form or other will
work against any password based authentication scheme, regardless of
the crytographic protocol used. So for example, zero-knowledge or
PAKE schemes, though making use of elegant cryptographic protocols,
remain as vulnerable to what is clearly the most common exploit seen
when it comes to passwords. HOBA is however not vulnerable to
database theft.
Appendix B. Example
The following values show an example of HOBA-http authentication to
the origin https://example.com:443. Carriage-returns have been added
and need to be removed to validate the example.
Public Key:
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAviE8fMrGIPZN9up94M28
6o38B99fsz5cUqYHXXJlnHIi6gGKjqLgn3P7n4snUSQswLExrkhSr0TPhRDuPH_t
fXLKLBbh17ofB7t7shnPKxmyZ69hCLbe7pB1HvaBzTxPC2KOqskDiDBOQ6-JLHQ8
egXB14W-641RQt0CsC5nXzo92kPCdV4NZ45MW0ws3twCIUDCH0nibIG9SorrBbCl
DPHQZS5Dk5pgS7P5hrAr634Zn4bzXhUnm7cON2x4rv83oqB3lRqjF4T9exEMyZBS
L26m5KbK860uSOKywI0xp4ymnHMc6Led5qfEMnJC9PEI90tIMcgdHrmdHC_vpldG
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DQIDAQAB
-----END PUBLIC KEY-----
Origin: https://example.com:443
Key Identifier: vesscamS2Kze4FFOg3e2UyCJPhuQ6_3_gzN-k_L6t3w
Challenge: pUE77w0LylHypHKhBqAiQHuGC751GiOVv4/7pSlo9jc=
Nonce: Pm3yUW-sW5Q
Signature:
VD-0LGVBVEVjfq4xEd35FjnOrIqzJ2OQMx5w8E52dgVvxFD6R0ryEsHcD31ykh0i
4YIzIHXirx7bE4x9yP-9fMBCEwnHJsYwYQhfRpmScwAz-Ih1Hn4yORTb-U66miUz
q04ZgTHm4jAj45afU20wYpGXY2r3W-FRKc6J6Glv_zI_ROghERalxgXG-QVGZrKP
tG0V593Yf9IPnFSpLyW6fnxscCMWUA9T-4NjMdypI-Ze4HsC9J06tRTOunQdofr9
6ZJ2i9LE6uKSUDLCD2oeEeSEvUR--4OGtrgjzYysHZkdVSxAi7OoQBK34EUWg9kI
S13qQA43m4IMExkbApqrSg
Authorization Header:
Authorization: HOBA result="vesscamS2Kze4FFOg3e2UyCJPhuQ6_3_gzN-
k_L6t3w.pUE77w0LylHypHKhBqAiQHuGC751GiOVv4/7pSlo9jc=.Pm3yUW-sW5Q
.VD-0LGVBVEVjfq4xEd35FjnOrIqzJ2OQMx5w8E52dgVvxFD6R0ryEsHcD31ykh0
i4YIzIHXirx7bE4x9yP-9fMBCEwnHJsYwYQhfRpmScwAz-Ih1Hn4yORTb-U66miU
zq04ZgTHm4jAj45afU20wYpGXY2r3W-FRKc6J6Glv_zI_ROghERalxgXG-QVGZrK
PtG0V593Yf9IPnFSpLyW6fnxscCMWUA9T-4NjMdypI-Ze4HsC9J06tRTOunQdofr
96ZJ2i9LE6uKSUDLCD2oeEeSEvUR--4OGtrgjzYysHZkdVSxAi7OoQBK34EUWg9k
IS13qQA43m4IMExkbApqrSg"
Authors' Addresses
Stephen Farrell
Trinity College Dublin
Dublin 2
Ireland
Phone: +353-1-896-2354
Email: stephen.farrell@cs.tcd.ie
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
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Michael Thomas
Phresheez
Email: mike@phresheez.com
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